The invention is in the field of electronic reproduction technology and is directed to a method for seamless engraving of patterns with an electronic engraving machine, a proofing unit for a seamlessly engraved pattern and an electronic engraving machine having such a proofing unit.
In an electronic engraving machine, an engraving element with an engraving stylus as a cutting tool moves in an axial direction along a rotating printing cylinder. The engraving stylus controlled by an engraving control signal cuts a sequence of cups arranged in an engraving raster into the generated surface of the printing cylinder. The engraving control signal is generated by superimposition of image signal values that represent the gradations between xe2x80x9clightxe2x80x9d and xe2x80x9cdarkxe2x80x9d to be engraved and a periodic raster signal for producing the engraving raster.
In four-color printing, a color set of four printing cylinders is engraved for the inks xe2x80x9ccyanxe2x80x9d, xe2x80x9cmagentaxe2x80x9d, xe2x80x9cyellowxe2x80x9d and xe2x80x9cblackxe2x80x9d; in packaging and decorative printing, a color set often comprises additional printing cylinders for special colors.
In practice, checking the engraved printing cylinders of a color set with respect to engraving quality and cylinder layout occurs with the assistance of a proofing device, for example in the form of a color monitor on which a color proof (soft proof) is displayed, or in the form of a color printer with which a color proof (hard proof) is printed on a sheet.
In decorative or packaging printing, an endless pattern, for example in the form of a wood grain, is often printed. In this case, the printing cylinder of the color set is engraved with a seamless pattern in the electronic engraving machine, said pattern repeating without a stop on the print medium during printing after a respective revolution of the printing cylinder in the printing press. A pattern has been seamlessly engraved when the seam at the start and end of the pattern meet at the circumference of the printing cylinder is invisible to the human eye.
Traditional proofing devices are not configured for evaluating the seam of an engraved pattern, since only the development of the engraved pattern with an upper edge and lower edge but not with the seam of interest is respectively displayed on the color monitor or printed on the printed sheet.
It is therefore an object of the present invention to improve a method for the seamless engraving of a pattern with an electronic engraving machine, a proofing unit for a seamlessly engraved pattern and an electronic engraving machine having such a proofing unit such that, in particular, the quality of the seam of an engraved pattern can be evaluated.
According to the present invention, a method and apparatus is provided for seamless engraving of a pattern in an electronic engraving machine. With an engraving element connected to an engraving control signal, a sequence of cups are engraved into a rotating printing cylinder and arranged in an engraving pattern. The engraving occurs line-by-line. The engraving control signal is generated from engraving data that represent gradations of the pattern to be engraved and from a periodic raster signal for generating the engraving raster. For planar engraving of the pattern, the engraving element executes an axial feed motion along the printing cylinder. The pattern is engraved such that a seam at which an upper edge and a lower edge of the pattern meet at a circumference of the printing cylinder is as invisible as possible. For evaluating the seam of the engraved pattern, a color proof of at least one separation color of the pattern is produced from the engraving data in that the engraved pattern is offset in the circumferential direction of the printing cylinder so that the seam lies within a visible area of the color proof.